Distillation and absorption are very common separation techniques used in the process industries. Both techniques require a large amount of energy due to the heating and cooling involved in the process. These techniques were mostly designed more than a decade ago when the economic, political and societal scenarios were different. It would be desirable to reduce the energy cost in the interest of economy as well as society.
There are complex separation configurations available that offer substantial reduction in energy and capital expenditures. These options include dividing wall columns. Dividing wall columns are normally used in distillation processes. FIG. 1 represents the prior art. The column operates at 390 psig with an overhead temperature of 95° F. This conventional depropanizer aims at separating C2/C3 components as an overhead product and C4/C4+ components as the bottoms product of the column.
However, the prior art possesses several disadvantages. It is not possible to condense the lighter components in the overhead product at 350 psig and using cooling water as the overhead cooling media. The overhead system has a partial condenser. C2 and lighter components (used as fuel gas) are drawn as the vapor product from the partial condenser. C3 product is the liquid stream from the partial condenser. A considerable amount of C3 components are lost to the fuel gas vapor stream. The loss of C3 components can be prevented by decreasing the overhead temperature (e.g., by using refrigeration) or increasing the column pressure. However, this increases the operating cost of the column. Although the prior art systems offers a sharp split between C3 and C3+, the recoveries of these components is low as they are lost in the fuel gas vapor stream.
FIG. 2 represents alternative prior art. This process uses a two column design. The first column is a reboiled absorber which aims at separating non-condensed products from the feed. The non condensables (used as fuel gas) are the overhead vapor product from this column. The bottoms from the first column are fed to the second column. C3 product is withdrawn as an overhead product in the second column and C4/C4+ is withdrawn as the bottoms product. Although, this prior art system offers high C3/C3+ recoveries. The disadvantage is higher capital expenditure and higher energy consumption.
Thus, it would be desirable to have a system that overcomes the disadvantages of the prior art systems.